The present invention relates to a porous polytetrafluoroethylene (hereinafter referred to as "PTFE") molded article, and a composite PTFE molded article and a highly densified PTFE sheet which are obtained from the porous PTFE molded article, and particularly relates to the porous PTFE molded article having excellent chemical resistance and excellent durability to deformation in a wide range of temperature, especially under the conditions of high temperature and high load, and the processed (densified) article obtained therefrom.
Since the porous PTFE molded articles have excellent chemical resistance and high tensile strength, they have been used suitably for many applications such as sealing materials, gaskets, etc. for processing equipment and pipings in the fields of chemicals, foods, and semiconductors; filters for filtrating various gases and liquids; clothes, sheets for medical use, etc. having gas permeability, water-impermeability and water repelling property.
Such porous PTFE molded articles are well-known. Particularly U.S. Pat. No. 4,598,011 discloses a porous PTFE molded article having a high matrix tensile strength, a relatively large pore size and a large porosity. The porous PTFE molded article disclosed therein is a molded article comprising a porous material having a microstructure characterized by nodes interconnected by fibrils and has an average MTS value of not less than 15,000 psi (hereinafter referred to as "Molded Article a"); a molded article having average MTS and C.I. values within the range surrounded by the point A (MTS=3,000, C.I.=0.4), the point B (MTS=12,000, C.I.=0.4), the point C (MTS=16,000, C.I.=0.2) and the point D (MTS=25,000, C.I.=0.2) in FIG. 1 (corresponding to FIG. 3 of the above-mentioned US patent) described later (hereinafter referred to as "Molded Article b"); or a molded article having an average MTS of not less than 3,000 and EBP of not more than 4 (hereinafter referred to as "Molded Article c"). The meanings of MTS, C.I. and EBP are explained later.
The above-mentioned porous PTFE molded articles have more enhanced tensile strength, cold flow resistance and porosity as compared with conventional ones, but in case of the use for gasket which is one of main applications of the porous PTFE molded articles, there arise the following problems.
1 Enough sealing effect cannot be obtained unless a relatively high stress or torque is applied for sealing.
2 In case of use under high stress, partial deformation in a specific direction occurs due to anisotropic cold flow. Particularly in case of use at high temperature, deformation in a specific direction becomes remarkably large.
An object of the present invention is to solve the above-mentioned problems and to provide a porous PTFE molded article which is less anisotropically deformable even at high temperature and high load, and if deformed, the deformed direction is not specific.
Another object of the present invention is to provide a composite molded article produced by laminating the porous PTFE molded article and other material.
Other object of the present invention is to provide a highly densified PTFE molded article or sheet which is produced by pressing and then sintering the porous PTFE molded article or sheet.